1. Field of the Invention
This invention relates to inverters.
More particularly, the invention relates to inverters of the kind comprising a resonant circuit including a capacitor and an inductor, a first main switching device arranged to conduct current from a d.c. source in one direction through the capacitor and inductor, a second main switching device arranged to conduct current from the d.c. source through the capacitor and inductor in the opposite direction, and drive means for causing the switching devices to conduct alternately, the drive means comprising a transformer having a primary winding in series with the capacitor and inductor and two secondary windings arranged to provide drive signals to the respective main switching devices.
2. Description of Related Art
An inverter of this kind is described in published European patent application EP-A-0 121 917. The described inverter operates at the resonant frequency of the resonant circuit, the main switching devices being switched at the zero crossing points of the oscillatory current in the resonant circuit, and there is no means of controlling the power output of the inverter.
A paper by Sebastian et al, entitled "Regulating Self-Oscillating Resonant Converters", 2nd European Conference on Power Electronics and Applications held at Grenoble, France, on 22-24 Sep., 1987 describes an inverter in which switching of the field-effect transistors forming the main switching devices is effected by two saturable transformers with primaries connected in the resonant circuit and secondaries connected to the gates of the field-effect transistors, so that the drive to each transistor is removed when the associated transformer saturates. Each transformer has an additional winding through which a variable controlled d.c. current is fed, to control the point at which the transformer saturates. The switching of the transistors can thereby be controlled to control the phase difference between the voltage and current in the resonant circuit so as to control the power output of the inverter. However, the described inverter is relatively complex.